1. Field of the Invention
This invention is generally directed toward a surgical bioimplant. Particularly, the invention relates to the geometry of the bioimplant's load bearing surfaces configured to improve resistance of the bioimplant to omnidirectional translations at the surgical site.
2. Description of the Prior Art
Spinal fusion is directed to provide stabilization of the spinal column for painful spinal motion and disorders such as structural deformity, traumatic instability, degenerative instability, and post-resection instability. Fusion, or arthrodesis, is achieved by the formation of an osseous bridge between adjacent motion segments. This can be accomplished within the disc space, anteriorly between contiguous vertebral bodies or posteriorly between consecutive transverse processes, laminae or other posterior aspects of the vertebrae.
A fusion or arthrodesis procedure is often performed to treat an anomaly involving an intervertebral disc. Intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae and cushion vertebral bodies. A normal intervertebral disc includes a semi-gelatinous component, the nucleus pulposus, which is surrounded and confined by an outer, fibrous ring called the annulus fibrosis. In a healthy, undamaged spine, the annulus fibrosis prevents the nucleus pulposus from protruding outside the disc space.
Spinal discs may be displaced or damaged due to trauma, disease or aging. Disruption of the annulus fibrosis allows the nucleus pulposus to protrude into the vertebral canal; a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on the spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Intervertebral discs may also deteriorate due to the normal aging process or disease. As a disc dehydrates and hardens, the disc space height will be reduced leading to instability of the spine, decreased mobility and pain.
Sometimes the only relief from the symptoms of these conditions is a discectomy, or surgical removal of a portion or all of an intervertebral disc followed by fusion of the adjacent vertebrae. The removal of the damaged or unhealthy disc will allow the disc space to collapse. Collapse of the disc space can cause instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage, in addition to severe pain. Pain relief via discectomy and arthrodesis requires preservation of the disc space and eventual fusion of the affected motion segments.
One of numerous solutions to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, preferably without the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable implant that could be used to replace a damaged disc and maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. To be successful the implant must provide temporary support and allow bone ingrowth. Success of the discectomy and fusion procedure requires the development of a contiguous growth of bone to create a solid mass because the implant may not withstand the cyclic spinal loads for the life of the patient.
Many attempts to restore the intervertebral disc space after removal of the disc have relied on various bone grafts promoting osteogenesis. The use of autograpft bone (taken from the patient), allograft bone (obtained from other individual) or xenograft (bone of a different species) is well known in both human and veterinary medicine. Both allograft and autograft are biological materials which are replaced over time with the patient's own bone, via the process of creeping substitution. Over time, a bone graft virtually disappears unlike a metal implant, which persists long after its useful life. Stress shielding is avoided because bone grafts have a similar modulus of elasticity as the surrounding bone. Regardless of the type of the bone graft, it should have the following characteristics:                Tolerance to high bearing loads; and        Osteoinductivity and osteoconductivity needed for accelerating the growth of new bone tissue at the site.Hence, compositionally, an implant advantageously has a substantial inner portion of mineralized bone and an outer portion or layer of demineralized bone providing a fusing interface with adjacent vertebrae.        
However, the composition of the implant alone is not necessarily sufficient to provide a high rate of fusion. Rather, the combination of composition and geometry of the implant markedly improves its biomechanical properties. Once in situ, the osteogenic implant is exposed to multidirectional compressive loads tending to cause the implant to translate, which, in turn, may cause neural and vascular injury, as well as collapse of the disc space. Accordingly, it is imperative that the coupling between the implant and the adjacent vertebrae remain structurally sound to minimize slippage and potential expulsion of the implant. One of the consequences of relative displacement is associated with a friction between the juxtaposed surfaces of the implant and the adjacent vertebral bodies, which leads to gradual, but not uniform thinning of the demineralized layer and, thus, detrimentally affects osteoinductivity and/or osteconductivity. Thus, among others, the design of the bioimplant should consider the following aspects:
Minimization of relative displacement between the implant and adjacent vertebrae caused by multidirectional compressive forces; and
Geometry of the load bearing surfaces of the implant should minimize damage to the surface features of the implant when the implant is exposed to multidirectional translations.
These problems have been addressed, but not fully solved, by providing osteogenic implants with texturized demineralized layers. For example, U.S. Pat. No. 6,511,509 discloses a texturized bioimplant having one or more texturized bone surfaces each provided with spaced or continuous protrusions. Configured uniformly, the protrusions engage the end plate(s) of the adjacent vertebrae at a uniform angle with respect to the central axis of the implant.
U.S. Pat. No. 6,511,509 represents a typical structure of a texturized surface. Regardless of numerous shapes and dimensions, the protrusions are typically uniformly shaped, dimensioned and oriented with respect to the endplate(s) of the vertebrae and are therefore nonselective in their response to compressive forces applied against them. However, such a configuration does not take into account the fact that applied compressive loads are typically multidirectional. Accordingly, whereas one group of protrusions, for example protrusions facing the end plate of the superior vertebra, may reliably anchor the implant in one direction, the other group of protrusions located on the bottom of the implant may be not as effective. Furthermore, the pressure distribution over the entire load bearing surfaces of the implant is rarely uniform. As a consequence, some of the protrusions are exposed to higher friction forces which have been found to lead to uneven scrapping of the demineralized layer located on the outer surface of the protrusions. Hence, the osteoinductivity of the bioimplant may be detrimentally affected.
Thus, a need exists for a bioimplant having a structure configured to minimize the displacement between the bioimplant and the adjacent vertebrae and to minimize damage to the demineralized layer of the bioimplant if and when such translations occur.